Method of and apparatus for the low-temperature liquefied gas

ABSTRACT

A system for the transportation of liquefied gas, e.g. in a methane-carrying tankship, in which the storage space is provided with a multiplicity of upright liquefied-gas vessels in which a quantity of the liquefied gas is retained during the return trip and the upper portions of the tanks are cooled by a cooling system in addition to the cooling afforded by the retained liquefied medium.

United States atent Becker 1 1 May 13, 1975 [54] METHOD OF AND APPARATUS FOR THE 2,687,618 8/1954 Bergstrom 62/50 PE T E 2,897,658 8/1959 BCCkWltll 1 62/55 LOW TEM RA UR Q 3,030,780 4/l962 Loveday 62/51 X Inventor: Rudolf Becker, ich. rmany 3,319.433 5 1967 Pauliukonis 62/45 3,392,537 7/1968 Woerner 62/50 [73] Assgneci Lnde Aktengesellschaf" 3 698,200 10/1972 Johnson et al .1 62/45 Wiesbuden, Germany [22] Filed: 177 I973 Primary E.raminerMeyer Perlin [21] APPL 425 31 Assistant Examiner-Ronald C. Capossela Attorney, Agent, or FirmKarl F. Ross; Herbert Dubno [30] Foreign Application Priority Data Dec. 22, 1972 Germany 2263063 Nov. 12. 1973 Germany 2356395 [57] A T A system for the transportation of liquefied gas, e.g. in [52] us CL 62/55; 62/240; 14/74 A; a methane-carrying tankship, in which the storage 220/9 LG space is provided with a multiplicity of upright lique- [5 i] 31. Cl. F17C 7/02 fi g vessels i which a q y of the quefied g [58] Field of Search 62/45, 50, 55, 51, 240; is retained during the return trip and the upper pop [14/74 A; 220/9 LG tions of the tanks are cooled by a cooling system in addition to the cooling afforded by the retained lique- [56] References Cited fled medium UNlTED STATES PATENTS 334,481 1/1886 Sone 114 74 A 10 Clams 6 Drawmg F'gures PATENTED 3,882,885

SHEET 10F 3 FIG. FIG. 2

PATENTEU HAY 1 31975 SHEET 8 BF 3 PATENTED HAY 1 31975 SHEET 3 OF METHOD OF AND APPARATUS FOR THE LOW-TEMPERATURE LIQUEFIED GAS FIELD OF THE INVENTION The present invention relates to a method of and an apparatus for the transport of liquefied gases at temperatures below their boiling point and especially such lowtemperaturc liquefied gases as liquefied methane or natural gas; more particularly, the invention relates to the transport of such low-temperature liquids in an environment at a temperature above the boiling point of the liquid, with vertically elongated vessels or receptacles disposed in a storage space. generally the hold of a tanker or other ocean-going conveyance.

BACKGROUND OF THE INVENTION As has been described in the aforementioned copending applications, it has been found to be advantageous to supply regions poor in fuel and petrochemical starting materials with liquefied methane or natural gas obtained from regions rich in such products by liquefaction of the gas at the extraction location, transport of the liquefied gas at temperatures below its boiling point via ocean-going vessels such as tank ships, and storage of the liquefied gas followed by regasification at a distribution or consumption site.

It has been proposed, more specifically, to transport liquefied natural gas in the thermally insulated hold of a tank ship or like ocean-going vessel by providing within this closed storage compartment a multiplicity of upright and generally vertically elongated vessels or receptacles which may be charged and discharged in succession, in groups or in parallel with one another. In general, the pressure in such vessels remains at ambience or only slightly thereabove during the transport and may be raised to a somewhat higher pressure when the liquefied gas is discharged, eg by applying pressure above the level of liquid in the receptacle to drive the liquefied gas upwardly and outwardly through a tube reaching towards the bottom of the vessel or receptacle.

Following discharge a limited quanity of the liquefied gas is frequently retained in the bottom of each vessel or receptacle to serve as a cooling medium for the vessel so that recooling of the system is not required when the receptacles are to be charged with the liquefied gas at the filling station.

While the retention of a small quantity of the liquefied gas required that a corresponding proportion of the receptacle be unavailable for delivery capacity, the loss in capacity is outweighed by the saving of energy and gas which would otherwise be necessary to bring the system down to a suitable temperature. Furthermore, fluctuations in the temperature of the vessel are not desirable because of the thermal stresses created thereby and the difficulties resulting from expansion and contraction of the parts of the chambers or the individual vessels and the duct systems provided therefore.

Thus, as a practical matter, it has been found to be advantageous to retain a small quantity of the liquefied gas at a temperature flow below its boiling point in the bottom of an upright vessel which has a width and even a girth in the horizontal plane which can be small relative to its height or length.

However. with such upright and vertically elongated vessels, it has been found that there is an unavoidable penetration of heat into the insulated compartment and a consequent heating of the regions of the receptacle which are remote from the retained quantities of liquetied gas. In other words, the presence of the small quantities of retained liquefied gas at the bottom of each receptacle does not prevent warming of the upper portions at the tops of these receptacles by, for example, a convection current rising between the receptacles. The problem becomes more pronounced as the height of the tank is increased, and, of course, it is desirable to have tanks which are as tall as can be accommodated in the storage compartment of the ship. In short, the thermal conduction of the Walls of the individual compartments or receptacles and the heat-absorbing capacity of the retained volume of liquefied gas, does not insure maintenance of the upper portions of each receptacle and the space enclosed thereby at the desired low temperatures.

Consequently, upon filling of the vessels or receptacles at the source of the liquefied gas, a portion of the liquefied gas must be consumed in bringing the upper parts of the receptacles to the storage and transport temperatures. It has been found to be impractical to operate with receptacles of small height and in which the warming effect would be minimal, because this approach reduces the capacity of each vessel or receptacle, increases the number of receptacles which are required and consequently increases the proportion of the total load carrying capacity which must remain filled with the liquefied gas for the return trip as the cooling medium.

OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide an improved method of operating a multivessel storage compartment for the transport of low-temperature liquefied gases whereby the aforedescribed disadvantages may be obviated and vaporization losses on filling the vessels can be reduced.

It is another object of the invention to provide a system for the transport of liquefied gas, especially liquefied natural gas, whereby the aforedescribed disadvantages are obviated and improved efficiency can be attained with low loss.

It is still another object of the invention to provide a method of and an apparatus for the transport of a lowboiling liquefied gas in individual receptacles or vessels in an insulated compartment which is capable of holding the evaporation loss on filling to a minimum and yet is effective with relatively tall vessels,

SUMMARY OF THE INVENTION The above and other objects of the invention are attained with a system in which at least the return transport of the storage system is carried out under conditions in which the primarily empty storage vessels are cooled with a cooling system effective at the upper portions of these vessels.

More particularly, the invention comprises a method of operating a system for the transportation of liquefied gases at temperatures below their boiling point which comprises filling a multiplicity of vertically extending storage vessels in a thermally insulated cargo compartment of a conveyor with liquefied gas at or below the transport temperature, transporting the liquefied gas by displacing the conveyance from the filling site to the discharge site, primarily emptying each of the vessels or compartments at the discharge site while retaining in each compartment a minor quantity of the liquefied gas to maintain a relatively low temperature during the return transport of the conveyance, returning the conveyance from the other discharge site to the charging site, and cooling the upper portions of the vessels during the return passage using a cooling arrangement or system specifically designed to prevent the development of large temperature gradients between the upper portion of each tan (cooled by the auxiliary system) and the lower portions of each tank cooled by the residual liquefied gas. V

The heat which penetrates from the exterior to the upright vessels during the return transport of the conveyance generates in the regionof each vessel an upwardly moving convection current which promotes heating of the container in its upper regions. According to the invention, this upwardly moving heat is dissipated or removed so that the upright vessels remain cold even during their return transport so that evaporation and concommitment energy loss, upon the next filling of the upright vessel, is held to a minimum.

The cooling system according to the invention may be a conventional cooling circuit having a coolant (refrigerant) in heat exchanging relationship with. the upper portion of the upright vessel The coolant can be a deep-cooled gas or a supercooled fluid which is heated in contact with or in heat-exchanging relationship with the upper portion of the upright vessel, but most advantageously is a cooled liquid which is evaporated in heat-exchanging relationship with these upper portions. ln the latter case, the heat abstracted by the. refrigerant or coolant is made upof part ofthe latent heat of evaporation abstracted upon transformation of the coolant liquid to gas.

It has been found to be especially economical for the coolant to be constituted at least in part by the low.-

boiling liquefied gas transported by the system and es-.

pecially when all of the coolant is constituted thereby. Of course, another coolant or refrigerant can be provided in addition to this low-boiling liquefied gas.

According to a further feature of the invention, the cooling system is provided in the form of one or more vessels which are at least partly filled with the coolant or are traversed thereby, the auxiliary receptacles being disposed above the upright liquefied-gas storage vessels. These auxiliary receptacles provide heatexchange surfaces above the storage compartment, abstract heat from the rising convection gas streams.

The geometric form of the auxiliary receptacles is also important. When the compartment is provided with a plurality of upright liquefied-gas storage vessels,

. it is preferred to provide the auxiliary vessels with'a greater horizontal dimension than vertical dimension and to dispose these auxiliary receptacles in a prone position.

The filling of the prone vessel or vessels with a portion of the low-boiling liquefied gas can be carried out simply during the filling or emptying of the upright liquefied-gas storage vessels. lnthis case the system need not be provided with additional devices such as pumps or blowers to displace the liquid into the prone vessels since the apparatus provided for filling and emptying the storage vessels can be used directly for this purpose.

An apparatus for carrying out the method of the present invention can comprise a multiplicity ofupright liquefied-gas storage vessels within a thermally' in'su lated storage compartment and, also disposed in the latter, auxiliary receptacles with horizontal dimensions greater than their vertical dimension and a generally in 5 prone orientation, the auxiliary receptacles being con nected via conduits with the lower portion of at least i one upright storage vessel. In other words, at leastone of the storage vessels is provided with a dipor siphon tube which reaches downwardly into the vessel, preferably to the surface of the residual liquefied gas therein and communicating at its upper end withone or more of the auxiliary cooling receptacles disposed above the L bank of storage vessels.

therein to drive the liquid out of the vessel via a dipor' riser tube, whereas in prior systems these tubes communicate with a main conduit and extend via other ducts to a land site. In the system of the present inven; tion, at least one of these storage vessels is emptied Q through one or more of these auxiliary cooling recepta the liquid now contained in the prone auxiliary receptacle cannot return to the upright storage vesselfrorn which it was displaced. This conduit preferably'com municates with the central region of the prone auxili- 40 ary-cooling receptacle so that an automatic control,

amountwhich fills a prone receptacle level.

gion, ofa prone vessel need not be the precisegeornetrical center but is a region, intermediate the upper and lower wall, in whichthe upstanding riser tube termii nates and which corresponds to the quantity of liq'ue-fl fled gas determined to be advantageous for the auxiliary cooling process. i

.When a multiplicity of upright storage vesselsis, provided within the compartment, it has been found to be advantageous to provide a number of cold-storage.

members, i.e. a multiplicity of prone receptacles disposed above the respective upright vessels; The'interit mediate regions of at least several of the prone receptai cles are connected with the lower regions of respective upright vessels. The intermediate regions of the r cooling during the return trip.

The connection of the prone auxiliary-cooling recepr tacles with the upright storage vessels, according tot'he present invention, serves to cool the storage vessels" I with low-boiling liquefied gas at least partly displaced f the emptying Asdescribed in the aforementioned copending appli cles, i.e. the liquefied gas passes first into the auxiliary receptacle and then is displaced toward the land stor-' age or regasification site. Since the. upright vessel is emptied, the clip tube connecting it with theauxiliary' storage vessel is closed, e.g. by means of a valve, so that without additional. valves, may be provided, .to insure' 5 that sufficient coolant will be retained in the prone on I I i auxiliary cooling receptacle. This. quantity is the to'its median In this connection it should be noted that middle re- BRIEF DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which FIGS. 1. through 6 are flow diagrams illustrating various embodiments of the present invention.

SPECIFIC DESCRIPTION In FIG. 1 I have shown an embodiment of the invention, for a tankship of the type generally described in the aforementioned copending application and which thus comprises a thermally insulated cargo-carrying compartment having a multiplicity of upright liquefiedgas storage vessels or tanks close spaced apart and formed with means for filling and emptying the tanks. Neither the compartment nor such means has been shown in detail in the present case and it will be understood that the compartment and the charging and discharging means can be of the type described in the aforementioned copending applications. The upright liquefied-gas storage vessels are connected in rows A and B which extend horizontally (perpendicular to the plane of the pipe in FIG. 1), each row being associated with a prone receptacle 2 or 3. In the illustrated embodiment, row A of the upright gas-storage vessels 1 is disposed immediately below the horizontal tank or receptacle 2 while row B of the storage vessels 1 is disposed immediately below the horizontal tank 3. As also has been shown in FIG. 1, the width w of each tank 1 perpendicular to the direction in which the row extends is a small fraction of the length L of the vessel 1 and preferably the length lies vertically. The diameter D of the horizontal tank 2 associated with each row is equal to the width w while the tanks 1 and 2 of each row have a common vertical plane P through their respective axes.

Riser tubes 4 and 5 extend from one or more storage vessels 1 of each row upwardly into the overlying horizontal receptacle 2 or 3 so that the end 4a of each of these tubes lies at a distance r above the bottom of the horizontal tank to determine the level a of liquefied gas to be retained therein. Similarly, the bottom end 4b of each of the riser tubes 4, 5, terminates slightly above the bottom of the vessel 1 and defines the level c of liquefied gas to be retained therein upon emptying.

At the beginning of the return trip of the tankship, the liquefied gas in each upright vessel 1 lies at the level b while the liquefied gas within the tanks 2 or 3 lies at the level a, substantially at the ends 412 and 4a of the tube 4. With continued evaporation during the return trip, the bodies of liquid at the base of the upright ves-' sel I maintain the lower portion of each storage vessel cool, i.e. substantially at the storage temperature of the liquefied gas, while convection heating is precluded by the cooling of the ambient gas around the tanks 2 and 3 by heat exchange through the walls thereof with gradual evaporation of the liquefied gas retained therein. Thus the upper portions of the storage vessels, 1 are likewise cooled. The vapors arising from such evaporation are vented via lines 6 and 7 communicating with the uppermost portions of the receptacles 2 and 3.

During filling of the vessels 1 at the gas-liquefaction site, liquefied gas may be introduced through a tube reaching to the bottom of each vessel and the gases thereabove vented in the manner described in the aforementioned copending application. Upon emptying of the vessels 1 at the gasification site with gas at an atmospheric pressure, e.g. 2 atms. is supplied to each of the storage vessels 1 above the liquid level therein, e.g. through a central gas duct 8 and the branch lines 9 and 10. The liquid is thereby displaced upwardly through the tubes 4 and 5 into the receptacles 2 and 3, thereby filling the latter and thereafter driving the liquid out of the system through lines 6 and 7 to the gasification plant or some other storage facility. Upon termination of the operation, lines 8 through 10 are vented to restore ambient pressure to each of the storage vessels l, whereupon the level of liquid in tanks 2 and 3 sinks to the level a, the liquefied gas returning to the storage vessels 1 serving to cool the bottoms of each of the latter. The level a of liquid thus remains for the beginning of the return trip.

When a cooling in the lower portion of the upright vessel 1 is not required, the lines 6 and 7 can extend downwardly substantially to the level a and thus eliminate any residual liquid above the level a and preclud ing the return of liquid from the horizontal receptacles 2 and 3 to the upright receptacles 1.

In FIG. 2, we show a modification in which rows F and G of upstanding, vertically elongated vessels 1] and 12 are provided as storage and transport vessels. The bottom portion of each vessel 11 is connected via line 13 to the upper portion of a vessel 12 of the neighboring row. A pipe 15 here rises to the level a at the center of a horizontal receptacle 14 overlying the row F of upstanding vessels 11 and communicates with the top of at least one of these vessels. Another pipe 17 rises to the level a within the horizontal receptacle 16, but reaches downwardly into a vessel 12 just short of the bottom thereof. Vent pipes 18 and 19 are provided from the tops of the horizontal receptacles 14 and 16.

During the filling and emptying of the system, the horizontal receptacles 14 and 16 are completely filled with the liquefied gas. During the filling of the upright vessels 11, 12 the liquefied gas is first fed via line 18 into the horizontal receptacle 16. Since this has been filled to the level a liquid passes through the tube 17 into upright vessel 12 or row G and then via lines 13 into upright vessel 11 of row F. Finally, the liquid reaches line 15 and fills the tank 14 to the level a I thereof. When the level a is reached in both of the horizontal receptacles l4 and 16, the filling process is terminated. During emptying of the system gas is forced under elevated pressure through line 19 and via line 15 into the top of receptacle 11. The liquid is displaced through line 13 and vessel 12 until the liquid is fully driven out of the system except for residual liquid at the base of each storage vessel 11, 12 and in the tanks 14 and 16 to level a therein. During the return passage of the ship, evaporation losses cause a drop in the liquefied gas in the horizontal tank to the level b. During the payload passage, liquid in the tanks 14 and 16 also evaporates to maintain cooling and then a drop in the level is replenished at discharge in the manner stated. In the system of FIG. 3, three rows of upright vessels 20 are provided at C, D and E, linked together as will be apparent hereinafter. Each row is provided with a horizontal receptacle 21, 22 or 23. In this embodiment, the horizontal receptacle 22 associated with the intermediate row D is connected at its horizontal median plane (level a) with tubes 24 and 26 dipping substantially to the bottom of the tanks or vessels 20 of the outer rows C and E. The region at the horizontal median plane of horizontal receptacle 21 is connected by a line 28 to the top of upright vessel ofrow D while another tube 27 connects this region to the top of a corresponding vessel of row C. Another line or tube 29 connects the top of a vessel 20 of row E with the intermediate region of the horizontal receptacle 23 associated with this row. During the return passage of the ship horizontal receptacle 21 and 23 contained liquefied gas to the level b and receptacle 22 contains liquefied gas to the level a. The wall of the uppermost portions of vessels 20 is effected by evaporation of liquid in the receptacles 21 through 23.

The system is charged by supplying liquid via line to the tank 22 from which the liquid, upon reaching the level a therein flows to the vessels 20 or rows C, D and E to drive gas from these vessels throughlines 27, 28 and 29 and lines 31 and 32. When the vessels 20 are filled, the liquid rises to the level ain the horizontal receptacles 21 and 23. The filling process is ended when the level a is attained in all of the horizontal receptacles.

During the payload passage, liquid evaporates from the receptacles 21 through 23 to bring the level of liquid therein to that represented at b.

During the emptying of the upright vessels 20, gas at an elevated pressure is supplied via lines 31 and 32 into the horizontal receptacles 21 and 23 and, via lines 27 and 29, to the upright vessels 20 of rows C and E. The liquid is driven through tubes 24 and 26 into the horizontal receptacle 22 and emerges at line 30. Similarly gas pressure is applied via line 28 to the top of the horizontal vessel 20 of row D and the liquid therein is driven through line 25 and receptacle 22 from the system via line 30. When the several upright vessels 20 are emptied to the extent that no further liquid is driven therefrom, the application of gas pressure is terminated at line 31 and 32' and the liquid in receptacle 22 above the level a, flows to each of the upright vessels 20 to form the sump liquid therein serving to cool the lower portions of each of these upright vessels. The liquid remains at the level a at the start of the return trip only in receptacle 22.

Because of the relatively simple system for filling the horizontal receptacles the arrangement of FIG. 3 has the important advantage that within the upright vessels substantially no buffer volume is provided about the liquid surfaces therein. A buffer volume is provided for the entire system above the liquid level a in receptacle 21 and 23 during the payload passage of the ship and hence the overall transport capacity of the ship ,is increased.

The principles of FIGS. 1 through 3 are also applied, as shown diagrammatically in FIG. 4, to five or more rows of upright vessels. In FIG. 4, the upright vessels 41 through 46 are provided in rows I-I-O, each with a respective horizontal receptacle 47-53 overlying the row. The alternate horizontal receptacles 48 and 51, each between a pair of other horizontal receptacles 47, 49 and 50, 52, respectively, are connected by lines 54, 55, 56 and 57, 58, 59 to the vessels 41-42 and 4345 as described for the three row assembly of FIG. 3. Upon loading of the tankship, all of the upright vessels are filled and liquefied gas attains the level a in all of the overlying receptacles 47 through 53 as described for I the system of FIG. 3.

To empty the system, gas pressure is applied via central gas-supply line 60 to the horizontal receptacle 47, 49 and 50, 52, thereby driving the, liquid outwardly from all of the upright vessels 4045 via the central receptacles 48 and 51; and, via line 61 to the top ofvessel 46. The liquid is driven from this vessel 46 via a line .621

leading to the horizontal receptacle 53 and then through line 63 to the land station. ,Upon suchempty 7 ing, the horizontal receptacles 47, 49, 50 and 52 con- ,tain liquid to the levels b while receptacles 48, 51 and I 53 retain liquid tothe level a. During thereturn pas sage, evaporation of liquid in thereceptacles maintains cooling of the upper portion of the upright vessel:

FIG. 4 shows how the principles of the invention can be extended practicallyto any number of rows'of up right vessels with any desired retention of the coolant in the horizontal receptacle. Only the change in the liquid level in vessel 46 need be monitored to determine 1 when emptying is complete. t 7

FIGS. 5 and 6 illustrate another embodiment of, the

invention in which individual rows of upright vessels may each be provided with two horizontal receptacles of different diameters. I

In FIG. 5, the horizontal receptacle 72 of each row I of upright vessels receives an inner receptacle 71 of 1 relatively small diameter. One of the outer receptacles 72 is of larger diameter. Both of the horizontalrecepta, I I

cles 71 and 72 are connected to the upright vessel .70, the connections including a line 73 opening at the level a and at the top of the upright vessel 70' and a line-74 opening into the vessel 71 and closeto the bottom of the upright vessel 70. The liquid in receptacle 72 is supplied during the filling of the tankship. Thus liquefied, A gas is directed via the inner receptacle 71 which herelh acts as a central supply duct, and passes via linev 74 to each of the upright vessels 70.The liquid flows vialine 73 into the outer receptacle 72 and, when liquid no I longer is fed to the system, the level falls to that represented at 0. During the payload passage, the liquid level advantage of this arrangement lies in the greater safety provided by enclosure of receptacle 71 inrecept acle I The embodimentiof FIG. 5 differs from that of FIG. 6 in that the immersion tube 74 here reaches to the level a of the outer, horizontal receptacle 82 and substantially to the bottom of the upright vessels, while the tube 83 communicates between the inner receptacle 81 and the top of the upright vessels 80. After emptying of the upright vessels 80, the discharging gasf blows through the system without disturbing the retained liquid at level a which is employed for, cooling 7 of the upper portion of the upright, vessels 80during the return passage.

I claim:

1. In the transport of a liquefied gas at a temperature below its boiling point in an insulated cargo compartment of a conveyance having a plurality of upright vertically elongated storage vessels disposed in said fcompartment with a horizontally extending receptacle overlying said vessels, connectedtheretoby a riser tube 5 and in heat-exchanging relationship with ambient gases around upper portions of said vessels between a location in which the liquefied gas is loaded into said vessels and at a location at which the liquefied gas is discharged from the vessels, the improvement which comprises the following steps:

a. maintaining in said vessels a level of said liquefied gas above the base of said vertically elongated storage vessels to cool the vessels to preclude the necessity of recooling of the vessels when the vessels are to be charged with the liquefied gas at the filling station;

b. filling the vertically elongated storage vessels from said level to their tops and filling the horizontally extending receptacle overlying said vessels to a location at an intermediate distance between the bottom and the top of said horizontal receptacle with the liquefied gas to form a liquid-gas interface at said location; and

c. maintaining the level of the liquid-interface in the horizontal receptacle substantially at said location to insure an adequate amount of the liquefied gas is available in said horizontal receptacle to effect heat exchange with said vertically elongated storage vessels by vaporization of the liquid-gas interface.

2. ln a liquefied-gas storage and transfer system com prising a thermally insulated compartment and a plurality of upright vertically elongated liquefied gas-storage vessels in said compartment, the improvement which comprises the following elements:

a. a horizontally extending receptacle overlying said vessels and in heat-exchanging relationship with ambient gases around upper portions of said vessels; and

b. a riser tube extending upwardly from a location near the bottom of one of said storage vessels into the horizontally extending receptacle and to a location in the interior thereof, said tube opening into the interior of said receptacle at a location above the bottom of the receptacle and as an intermediate distance between the bottom and the top of said horizontal receptacle to maintain a liquid-gas interface in said receptacle substantially at said intermediate distance between the bottom and top of said horizontal receptacle to cool said receptacle by va' porization at said interface.

3. The improvement defined in claim 2 wherein said riser includes a duct communication between said receptacle and a lower portion of at least one of said vessels.

4. The improvement defined in claim 3 wherein said duct opens substantially at a central portion of said receptacle between the top and bottom thereof.

5. The improvement defined in claim 3 wherein at least two such receptacles are provided and one of said receptacles is connected by said duct to a lower portion of said one of said vessels, said riser including a further duct connecting the other receptacle to an upper portion of one of said upright vessels.

6. The improvement defined in claim 3 wherein said vessels are arranged in at least one row, said receptacle overlying and extending along said row.

7. The improvement defined in claim 6 wherein a plurality of said rows are provided, each with a respective one of said receptacles, said riser including a respective duct connecting at least some of said receptacles with lower portions of the vessels of respective rows.

8. The improvement defined in claim 7 wherein said riser includes duct connecting to the receptacles to upper portions of the vessels of respective rows.

9. The improvement defined in claim 8 wherein said riser includes ducting connecting at least some of said receptacles to vessels of the respective row and other ducts connecting the same receptacles to the vessels of adjoining rows at bottom portions thereof.

10. The improvement defined in claim 8 wherein one of said receptacles is received in another of said recep- 

1. In the transport of a liquefied gas at a temperature below its boiling point in an insulated cargo compartment of a conveyance having a plurality of upright vertically elongated storage vessels disposed in said compartment with a horizontally extending receptacle overlying said vessels, connected thereto by a riser tube and in heat-exchanging relationship with ambient gases around upper portions of said vessels between a location in which the liquefied gas is loaded into said vessels and at a location at which the liquefied gas is discharged from the vessels, the improvement which comprises the following steps: a. maintaining in said vessels a level of said liquefied gas above the base of said vertically elongated storage vessels to cool the vessels to preclude the necessity of recooling of the vessels when the vessels are to be charged with the liquefied gas at the filling station; b. filling the vertically elongated storage vessels from said level to their tops and filling the horizontally extending receptacle overlying said vessels to a location at an intermediate distance between the bottom and the top of said horizontal receptacle with the liquefied gas to form a liquidgas interface at said location; and c. maintaining the level of the liquid-interface in the horizontal receptacle substantially at said location to insure an adequate amount of the liquefied gas is available in said horizontal receptacle to effect heat exchange with said vertically elongated storage vessels by vaporization of the liquid-gas interface.
 2. In a liquefied-gas storage and transfer system comprising a thermally insulated compartment and a plurality of upright vertically elongated liquefied gas-storage vessels in said compartment, the improvement which comprises the following elements: a. a horizontally extending receptacle overlying said vessels and in heat-exchanging relationship with ambient gases around upper portions of said vessels; and b. a riser tube extending upwardly from a location near the bottom of one of said storage vessels into the horizontally extending receptacle and to a location in the interior thereof, said tube opening into the interior of said receptacle at a location above the bottom of the receptacle and as an intermediate distance between the bottom and the top of said horizontal receptacle to maintain a liquid-gas interface in said receptacle substantially at said intermediate distance between the bottom and top of said horizontal receptacle to cool said receptacle by vaporization at said interface.
 3. The improvement defined in claim 2 wherein said riser includes a duct communication between said receptacle and a lower portion of at least one of said vessels.
 4. The improvement defined in claim 3 wherein said duct opens substantially at a central portion of said receptacle between the top and bottom thereof.
 5. The improvement defined in claim 3 wherein at least two such receptacles are provided and one of said receptacles is connected by said duct to a lower portion of said one of said vessels, said riser including a further duct connecting the other receptacle to an upper portion of one of said upright vessels.
 6. The improvement defined in claim 3 wherein said vessels are arranged in at least one row, said receptacle overlying and extending along said row.
 7. The improvement defined in claim 6 wherein a plurality of said rows are provided, each with a respective one of said receptacles, said riser including a respective duct connecting at least some of said receptacles with lower portions of the vessels of respective rows.
 8. The improvement defined in claim 7 wherein said riser includes duct connecting to the receptacles to upper portions of the vessels of respective rows.
 9. The improvement defined in claim 8 wherein said riser includes ducting connecting at least some of said receptacles to vessels of the respective row and other ducts connecting the same receptacles to the vessels of adjoining rows at bottom portions thereof.
 10. The improvement defined in claim 8 wherein one of said receptacles is received in another of said receptacles. 